Z-push fatal error when connecting through VPN

I am trying to do some testing of Kopano/z-push/Outlook with two different servers, but which should be identical installations of Kopano 8.5.8 and z-push 2.3.9. The purpose is to test the upgrade to 8.7.x. With my local server I can connect with Outlook by Active Sync. The second server is located miles away and I use OpenVPN to connect onto its LAN. When I try to connect with a new profile in Outlook, it always fails: